In-line consolidation of braided structures

ABSTRACT

A braided in-situ consolidated structure of fiber reinforced thermoplastic resin strands is formed by braiding the strands on a mandrel and then advancing the mandrel and the braided structure as it is formed continuously through a heating zone to heat the resin to a flowable condition then cooling the structure as it leaves the zone. Pressure may be applied or developed against the braided structure during its formation.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of U.S. application Ser. No.07/320,552 filed Mar. 8, 1989 now abandoned which in turn is a divisionof U.S. application Ser. No. 07/151,582 filed Feb. 2, 1988 and nowabandoned.

This invention relates to complex shaped structures and moreparticularly it relates to braided structures of fiber reinforcedthermoplastic strands.

Fiber-reinforced plastic structures have been used for many years withincreasing success because of their high strength, light weight and easeof fabrication compared to the wood or metal structures which theyreplace. Fibers such as glass, carbon, ceramic and aramid are popular asreinforcement, and thermoplastic resins are common polymeric matrices.

Braiding is one process for producing such structures and generallycomprises forming an array of yarns extending substantially parallel tothe axis of the structure and interlacing the yarns in a pattern throughthe array so they are interlaced with one another.

Polymeric materials reinforced with continuous filaments are used aprecursors for highly-stressed parts such as aerospace componentsrequiring the highest possible strength and stiffness with the lowestpossible weight. When a composite preform is made with both reinforcingfibers and a matrix material, it must be consolidated in a subsequentstep such as molding to form the final product. This consolidationprocess generally reduces the volume of the preform as air is removedand develops local crimp in the reinforcing fibers. Crimped fibersprovide less reinforcement than straight ones and thereby reduce thestrength and stiffness of the composite product.

SUMMARY OF THE INVENTION

When composite structures are made from fabric preforms which include athermoplastic matrix, the local fiber geometry may be distorted duringconsolidation. This occurs because the consolidation process uses heatand pressure to move the fibers and matrix into the space originallyoccupied by air. This action generally crimps the fibers, especially inthe case of thick walled structures. The problem can be overcome byeliminating the preform altogether and combining the operations of fiberplacement and consolidation. In this invention, braiding andconsolidation are combined using on-line heating to form low crimpthermoplastic matrix composites.

According to the present invention, an in-situ consolidated braidedcomposite structure (i.e., the ability to consolidate the structureduring braiding without the need of consolidation processing afterbraiding) is constructed from a plurality of lengths of fiber reinforcedthermoplastic resin strands braided on a mandrel. The strands areadvanced toward the mandrel and converged under tension in a braidingpattern around a location on the mandrel to form a braided structurethereon. The braided structure is advanced through a heating zone as thebraided structure is being formed where it is heated to a temperaturethat permits the thermoplastic resin to flow but below the degradationtemperature of the resin as the structure advances through the heatingzone. The braided structure is cooled as it leaves the zone. In analternate process, pressure may be applied by passing the braidedstructure through a heated die which is located in the region where theyarns converge. The die assembly is arranged to heat the incoming yarnsand soften the thermoplastic resin. Before entering the die, the braidtightens and squeezes the resin into the spaces between the fibers. Thenthe softened structure passes through the hot die to finalize thecross-sectional shape and surface. A cooling die may be added. Thepressure which develops depends on the size of die or type of componentsused. This in-line consolidation process provides a braided compositepart with a very low percentage of local crimp present in thereinforcing fibers of the part thus enhancing the stiffness and strengthof the product. Using the procedures described herein local crimp forcircular braided composite structures can be held to less than 3 percentor even less than 1 percent and for three dimensional braided compositestructures prepared by the two-filed Apr. 18, 1986, can be held to lessthan 10 percent or even less than 3 percent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view in perspective of circular braiding apparatususeful in practicing the invention and an associated consolidation unit.

FIG. 2 is a schematic view in perspective of another braiding apparatususeful in practicing the invention and an associated consolidation unit.

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 1.

FIG. 3a is a plan view of the adjustable heating chamber exit of theunit shown in FIG. 3.

FIG. 3b is an exploded view of the consolidation unit of FIG. 3.

FIG. 4 is a drawing of the consolidation unit of FIG. 2 partially brokenaway to show the relationship of the braided structure to the internalplates of the unit.

FIG. 4a is a plan view of the consolidation unit of FIG. 4.

FIGS. 5 and 6 are graphs of the projected path and reference path of onebraider yarn from the braided structure of Example II with in-lineconsolidation and in-line consolidation plus post consolidation,respectively.

FIG. 7 is a graph of the path and reference path of a single braidingyarn from the braided structure of Example II.

FIG. 7A is a schematic of the composite from Example III as prepared forconducting the test method for determining local crimp.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a circular braiding machine (manufactured byWardell Braiding Machine Co., Model No. W-38-53 equipped with #2carriers) generally designated 20 is fed from a creel 22 supportingsupply packages 24 of axial yarns 26 which are moved through guide tubes28. Braiding yarns 30 are fed from supply packages 32 mounted oncarriers 34 which are movable in face plate 36 (as indicated by arrows)by a horn gear mechanism (not shown) As shown in FIGS. 3, 3a and 3b,axial yarns 26 and braiding yarns 30 are braided over a shaped mandrel38 as heat is supplied in the form of heated nitrogen through nitrogeninjector ring 40 just before convergence point 42 which is surrounded byheating chamber 44. At the entrance to the heating chamber 44, thebraiding yarn array is guided from below by a funnel guide 46 and fromabove by guide ring 48. The funnel guide 46 is also used to minimize hotgas leakage. The nitrogen injector ring 40 has exit holes 50 directingthe flow of nitrogen gas which has been heated by nitrogen heater 52towards the braiding array. The heating chamber 44 is made of astainless steel cylinder wrapped with electrical heating coils 54 andfiberglass insulation 56. At the exit of heating chamber 44 is amechanical device 58 with an adjustable orifice 60. The orifice size isadjusted by rotating the angular position of a number of leaf elements62. The heating chamber wall temperature is measured by thermocouple 64and temperature of the interior of the heating chamber is measured bythermocouple 66.

Referring to FIG. 2, a 3D braiding machine (as disclosed in U.S. Pat.No. 4,719,837) generally designated 70 is fed from a creel 72 supportingsupply packages 74 of axial yarns 76 which are moved through guide tubes78. Braiding yarns 80 are fed from supply packages 82 mounted oncarriers 84 which are movable in grid support 86. The axial yarns 76 andbraiding yarn 80 form a braided structure 88 in specifiedcross-sectional shape. As this structure converges and tightens into itsfinal shape, it is heated by heating die assembly 90 and possibleauxiliary heating equipment. The thermoplastic resin flows and issqueezed between the fibers to fill the voids. The structure is thenpulled first through heating die 90 and then through cooling die 92 by apuller mechanism indicated by arrow 94.

As shown in FIGS. 4 and 4a, the heated die 90 which is positioned at theconvergence point of the braiding yarns is composed of a stationaryplate 96 and a moving plate 98. The stationary plate has a pair ofcartridge heaters 100 and a pair of thermocouples 101 incorporatedtherein. The moving plate 98 and the stationary plate 96 form an orifice102 which matches the specified cross-sectional shape of braidedstructure 88. The cooling die 92 is similarly composed of a stationaryplate 104 and a moving plate 106. Cooling coils (not shown) are insertedinto the stationary plate 104 of the cooling die in lieu of cartridgeheaters. Thermal insulation 108 is positioned between the two dies. Themoving plates in both dies may be vibrated by mechanical or hydraulicmeans (not shown) to vary the die pressure against the braided structure88 which lessens the pulling force requirement.

In operation, the dieless process described by FIGS. 1, 3, 3a, and 3b,braiding yarns and axial yarns are impregnated with a thermoplasticmatrix which matrix will soften and flow at a temperature which is belowthat which would cause the yarns to soften and flow. It is to be notedthat the braiding yarns and the axial yarns may differ in composition.The temperature imposed by the hot gas and the heating chamber must besufficient to cause the matrix to flow. The pressure which develops fromthe yarn tension and mandrel curvature must be adequate to fuse theyarns together.

In a heated die process, described by FIGS. 2, 4, and 4a, braiding yarnsand axial are similarly impregnated with a thermoplastic matrix. Thetemperature imposed by the heating die must be sufficient to melt thematrix only. The matrix should be softened before the yarns reach thedie so that it can flow into the interfiber spaces. The pressure exertedby the die must be adequate to consolidate the sample. The cooling diepreserves the imposed cross-sectional shape by solidifying the matrix.

Method for Determining Local Crimp Definitions

Braid--An interlaced fabric composed of braiding yarns and in some casesaxial yarns.

Axial Yarn--One of a number of yarns which pass lengthwise through thebraid and do not interlace each other.

Braider--One of a number of yarns which interlace each other and passthrough the braid in an irregular helix.

3D Braid--A braided multilayer fabric in which braiders pass completelythrough the thickness.

Braided Composite--A rigid structure made of a braid in which a largeportion of the spaces between the yarns are filled with a matrixmaterial.

The local-crimp can be defined as the fractional length differencebetween an actual yarn and a reference path. Usually this reference pathis taken to be a straight line oriented in the yarn direction. Incomplex shaped braided structures which can be layered or threedimensional (3D), the definition of a reference line must be generalizedto include all possibilities. For example, for circular braids, aprojected yarn path is measured for one circumference. This path couldbe quite circuitous in some cases even if the local crimp level is low.For this reason, the crimp is determined by comparing the length of theyarn projection to a smoothed reference curve. This reference curve iscomputed by taking a moving average of the projected yarn path. A windowwidth of 20% of the perimeter is used in this computation. Theprojection is taken on an irregular cylindrical surface which comes asclose as possible to the structure. By this procedure, the local crimpof any shaped part can be determined. Note that since the lengths aremeasured along a projected length, they are not the same as along theactual yarn.

For 3D braids, the crimp is determined by cutting a cross-section whichincludes a braiding yarn passing completely through the structure. Thelocal crimp is measured from the actual yarn length and a straight lineconnecting the end points.

Circular Braided Composites 1 Plot Graph of Projected Path of OneBraider

(a) Roll a transparent sheet around the composite. The sheet shouldremain in the form of an irregular cylinder or prism which comes asclose as possible to the composite.

(b) Mark the projected yarn path of one braider on the transparentsheet. The resulting graph shows the axial position "Y" vs.circumferential position "X" of the projected path. (Include 20% morethan one circumference and identify the points which correspond to thestart and end of one circumference or perimeter "P".) The prism orcylinder is then developed into a flat sheet where the path can bemeasured.

2. Plot a Smoothed Reference Path

(a) Compute the reference path from the projected yarn path at eachpoint by averaging "Y" over 20% of the circumference (from X-0.1*P toX+0.1*P).

3. Measure Path Lengths and Crimp

(a) Measure the lengths of the projected yarn path and the referencepath (designated Lp and Lr respectively).

(b) Compute the crimp (C) from

    (C=Lp/Lr-1)

3D Braided Composites (FIGS. 7, 7A) 1. Plot Path of Braider Through theThickness of a Composite

(a) Cut a cross-section of the composite 88 at an angle which includesthe path of a single braiding yarn passing completely through thematerial.

(b) Plot the path of the braider by plotting its in-plane position "Y"vs. its through-the-thickness position "X".

2. Plot the Reference Path

(a) Draw a straight line 88a through the sample thickness that connectsthe end points of the braider path.

3. Measure Path Lengths and Crimp

(a) Measure the lengths of the yarn path and the reference path(designated Lp and Lr respectively).

(b) Compute the crimp (C) from

    (C=Lp/Lr-1)

EXAMPLE I

A cylindrical tube structure is prepared by providing 16 groups of axialyarns and 32 braiding yarns of Kevlar™ 49 fiber (manufactured by E. I.du Pont de Nemours and Company) melt impregnated with Kodar™ PETGCopolyesters 673 (Eastman Chemical Products, Inc.) of total denier 4309where the fiber volume fraction is 0.5. The yarns are braided over acircular shaped mandrel, formed from solid aluminum, with a 0.5"diameter. The convergence half angle (angle between convergence cone andmachine centerline) is 66°. The heating chamber length is 4 inches andits diameter is 2 inches. The temperature of the nitrogen as suppliedwas 386° C. and the wall temperature of the chamber was 430° C. Thetemperature within the chamber was 370° C. The maximum temperaturebetween layers was determined to be 204° C. The axial velocity of thebraided structure was maintained at 5 in/min. The tension of both axialand braiding yarns was set to approximately 0.5 lb. After braiding alayer, the yarns were the resulting structure with the internal mandrelwas overbraided using the same process. A total of 5 layers were formed.

EXAMPLE II

A rectangular hollow circular braided structure was prepared under thesame conditions as Example I with the exceptions that a rectangularshaped mandrel formed from solid aluminum having the dimensions 0.65" by0.728" was used and that no axial yarns were provided. A total of 5layers were formed. Local crimp was determined to be 0.6 percent usingthe yarn path analysis as shown in FIG. 5.

The above on-line consolidated structure was further post consolidatedby heating at 220° C. for 15 minutes with a pressure of 425 psi. Localcrimp was determined to be 0.2 percent using the yarn path analysis asshown in FIG. 6.

EXAMPLE III

A rectangular slab is prepared by providing (1) 38 groups of axial yarnsof AS-4 carbon fibers (3KAS4W Hercules Magnamite™) melt impregnated withan amorphous polyamide, the method of which is described in Binnersley,et. al., U.S. Pat. No. 4,640,681, where the fiber volume fraction is 0.5and (2) 11 braiding yarns of Kevlar™ 49 fiber (manufactured by E. I. duPont de Nemours and Company) melt impregnated with the same polyamidecomposition as described for the axial yarns where the fiber volumetraction is 0.5. The total number of axial yarns provided is 502, eachyarn having a weight per length, including fiber and matrix, of 3300denier. The 11 braiding yarns have a weight per length, including fiberand matrix, of 4200 denier. The braided structure is prepared by thetwo-step process detailed in U.S. application Ser. No. 853,742 filedApr. 17, 1986. The temperature of the heated die was 300° C. and thecooling die was 104° C. The die pressure in both dies was 1256 psi;however, the pressure in the heated die was cycled between 0 and 1256psi by oscillating the moving plates with a hydraulic system. Becausethe yarns were manually manipulated, axial velocity of the braidedstructure was slow.

The above on-line consolidated structure was further post consolidatedto obtain full consolidation by heating at 310° C. for 20 minutes with apressure of 600 psi. Local crimp was determined to be 0.7 percent asdetermined by the yarn path analysis shown in FIGS. 7, 7A.

We claim:
 1. A method of building and consolidating a shaped compositestructure from fiber reinforced thermoplastic resin strands comprising:advancing said strands toward a heating die having an entrance and anexit; converging said strands under tension in a braiding pattern at theentrance to said die to form a braided shaped composite structure;heating said strands by means of said heating die as the strands areconverged under tension in said braiding pattern to a temperature whichwill cause the thermoplastic resin to soften and flow prior to formingsaid braided shaped composite structure; advancing said braided shapedstructure through said die while continuing to heat said braidedstructure within said die to cause continuing softening and flow as thestructure advances through said die, said temperature being below thedegradation temperature of the resin; applying pressure to said braidedshaped structure within said die to consolidate it, and cooling saidstructure after it leaves the heating die.
 2. The method of claim 1,including the additional step of applying pressure to said structurewhile cooling it to preserve the shape of said structure.